As a type of an industrial machine, there is known a cap fastener, an electrical screw driver, or a molding machine that moves a driving object to a predetermined position and then presses the driving object against a pressure object at a desired pressure. For an industrial machine of this type, a motor control method of executing a position control to cause the rotational position of a motor to follow a position command when the driving object moves, and for switching the position control to a feed-forward torque control to cause the motor to directly generate a set reference torque is frequently adopted.
Such a motor control method of switching between the position control and the feed-forward torque control has the following problems. When the control method is simply switched from one to the other, the discontinuous change in a torque during the switching generates an impact and a vibration or causes the excessive increase in a motor velocity.
To solve these problems, for example, Patent Literature 1 discloses a technique of switching between a position/velocity control using a position or a velocity command and the feed-forward torque control using reference torque based on a switching signal input from outside. According to this technique, when the motor velocity exceeds a velocity limit value in the torque control after the switching, the velocity control based on the velocity limit value is executed to prevent the motor velocity from increasing. Furthermore, when the motor velocity returns to fall within the velocity limit, the feed-forward torque control is executed again. Further, an appropriate setting of the value of an integrator included in a velocity controller during the switching can ensure torque continuity.
Patent Literature 2 discloses a technique of realizing a desired operation by feeding back an applied-pressure estimated value instead of using an applied pressure sensor in a robot controller such as a spot welding robot controller. Specifically, according to this technique, a control device includes a disturbance estimation observer that estimates a disturbance torque against a motor from a torque (a current) and a motor velocity, and a velocity controller (a velocity-loop processing unit). In a torque control mode (an applied-pressure control mode), the controller feeds back a deviation between a reference torque (an applied pressure command-converted torque) and an estimated disturbance torque. The estimated disturbance torque is output from the disturbance estimation observer to a velocity command. Furthermore, the robot controller presses a driving object against a pressure object in a state of a position control mode of executing an ordinary position control, determines a timing at which the estimated disturbance torque exceeds a predetermined value, and switches the position control mode to the torque control mode described above.